Head-Mounted-Displays (HMD) is a type of device with increasing popularity within the consumer electronics industry. HMDs, along with similar devices such as helmet-mounted displays, smart glasses, and virtual reality headsets, allow users to wear a display device such that the hardware remains fixed to their heads regardless of the person's movement.
When combined with environmental sensors such as cameras, accelerometers, gyroscopes, compasses, and light meters, HMDs can provide users with experiences in virtual reality and augmented reality. Virtual reality allows a user to be completely submerged into a virtual world where everything the user sees comes from the display device. On the other hand, devices that provide augmented reality allow users to optically see the environment. Images generated by the display device are added to the scene and may blend in with the environment.
One of the primary elements of HMDs is a display module mounted onto the head. However, since the unaided human eye cannot accommodate for images closer than a certain distance from the eye, eye piece lenses are required to re-image the display module such that the display appears to be at a comfortable viewing distance from the user. Such optical configuration requires lots of space between the eye piece and the display module. Furthermore, complex lenses are needed if the HMD needs to display images with high quality and wide field of view. These lenses often make the device very bulky to wear.
A number of methods had been invented to eliminate the need of heavy lenses in HMDs. Light field displays use a high resolution image panel with a microlens array to integrate subsets of images onto different parts of the retina. This method leads to images with low effective resolution. Retinal scanning displays are capable of producing images with resolution equivalent to the native resolution of the laser scanner. However, the stringent requirement to align the scanning mirror through the eye's pupil means that it is very difficult to fabricate an HMD that fits different anthropometric variations.
Holographic HMDs typically suffer from several problems. Firstly, image quality is typically poor as spatial light modulators (SLMs) are only available for either phase or amplitude modulation but not both. Computational holograms often suffer from what is known as the zero order which consists of light appearing in unwanted regions on the retina. Secondly, speckle is usually visible in holographic displays which use laser sources. Thirdly, an ideal holographic image requires using an SLM with very high resolution or small pixel size comparable to optical wavelengths. This also means holographic images would typically require very high computational load. Finally, image size or field of view (FoV) of a holographic display is typically inversely proportional to the pixel size of the SLM. Although pixel sizes of available of SLM technologies are getting smaller over years, they are still too large in the foreseeable future to produce large holographic virtual images.
US20090002787 (Adrian et al., published Jan. 1, 2009) references the use of an optical system to increase the size of the projected holographic image by diverging the light forming the displayed image. However, the use of lenses after the spatial light modulator would need to be large. This may increase the weight of the HMD to a similar weight as the conventional eye piece based HMD system.
U.S. Pat. No. 5,854,697A (Caulfield et al., issued Dec. 29, 1998) describes a waveguide hologram illuminator which includes a thin substrate and planar surfaces for transmitting light. The hologram is mounted on one planar surface to produce a holographic image with uniform spatial intensity characteristics over the region which spatial intensity modulation occurs. However, the patent does not solve the problems of small image size in holographic displays.
WO2014064228A1 (Tremblay et al., published Oct. 2, 2014) describes an illumination device which includes a substantially planar light guiding element used for illuminating spatial light modulators. One of the inventions uses several illumination sources to produce backlights of different angular spectrums. WO2012062681A1 (Fuetterer, published May 18, 2012) describes the use of an SLM to temporally multiplex segments of holographic images in different parts of space in order to create a larger image. This method requires stitching up of several discrete images and may lead to discontinuity in the overall image.
WO2014209244A1 (Urey, Published Dec. 31, 2014) describes a device that uses a pinhole imaging principle to achieve wide field of view. However, the described HMD device uses a matrix of micro reflectors attached to the back surface of an SLM which are subjected to a size limit of fabrication and alignment challenges. Pixelation from the mirror matrix is also likely to cause poor optical quality in the HMD, which adds up with pixel size limitations of the SLM itself.
WO2014151877A1 (Cizmar et al., published Sep. 25, 2014) describes a head mount display which uses an illumination beam that is not perfectly collimated to remove zero-order light by allowing the zero order to be defocused, spreading power across the retina. However, such methods would reduce the image contrast of the holograms.